1. Technical Field
The present invention relates to an A/D conversion circuit and an optical disk drive, and more particularly to a correction on asymmetry or vertical asymmetry of an input analogue signal.
2. Related Art
In a next-generation optical disk, such as an HD DVD or a BD (Blu-ray disc), a reproduced signal undergoes intersymbol interference from preceding and subsequent pits because of a relationship between pits in the disk and the diameter of a laser spot. Performing reproducing operation in accordance with a PRML (Partial Response Maximum Likelihood) scheme by active utilization of interference at the time of reproduction of a signal has hitherto been proposed. PRML is a signal processing scheme for reading data exhibiting the highest likelihood from a reproduced signal on the premise that interference exists between a preceding pit and a subsequent pit. The signal processing scheme is a combined technique including a PR detection technique for recording and reproducing data in a narrow frequency band, in an unmodified manner, without suppressing waveform interference and an ML decoding technique for decoding a bit string exhibiting the highest likelihood among a plurality of reproduced signal strings rather than making a determination on a per-bit basis.
A PR class of an HD DVD is determined to be PR [1, 2, 2, 2, 1] from a transmission characteristic achieved between a medium and a pickup. However, a modulation code of the shortest inversion 2T (T is a reference time length) is adopted, and there is a problem of the amplitude of a 2T signal being not acquired at PR [1, 2, 2, 2, 1]. A direct slicing method is usually used as a method for performing playback of an optical disk. When a pit length is 2T, a signal amplitude is small as mentioned above, and hence difficulty is encountered in extracting a 2T signal under the direct slicing method. Under the PRML method, a target signal having a time transition closest to a time transient of a reproduced signal is selected, and a bit string generating the target signal is output as a decoding result. The target signal is computed by means of convolution of a designated impulse response (a PR class) and a bit string. Consequently, an appropriate PR class corresponding to the recording density of the optical disk is selected. Provided that the PR class is expressed as PR[a, b], PR[1, 1] represents a characteristic of an impulse response appearing at two consecutive points of identification at a 1:1 ratio. Therefore, a response output for an input {0 . . 1 . . . } comes to {0 . . 1 1 . . . }. Moreover, PR[1, 2, 1] shows a characteristic of an impulse response appearing at three consecutive points of identification at a 1:2:1 ratio. Consequently, a response output for an input {0 . . 1 . . . } comes to {0 . . 1 2 1 . . . }. In the case of HD DVD, the PR class is expressed as PR[1, 2, 2, 2, 1] which shows that an impulse response appears at five consecutive points of identification at a 1:2:2:2:1 ratio.
However, under the PRML method, a vertically-symmetric reproduced signal can be decoded with high accuracy. However, the method is very unstable for a vertically-asymmetric reproduced signal and may sometimes adversely affect the signal.
JP 2002-279736 A describes detection of the degree of asymmetry of a signal, when the degree of asymmetry is large, binarization is performed through use of a slicer. When the degree of asymmetry is small, binarization is performed through use of a PRML.
JP 8-83403 A describes an adjustment circuit—which adjusts a center voltage level of a reference voltage—being provided to a flash-type A/D conversion circuit used in a channel for reproducing data of PRML scheme.
However, according to the technique disclosed in the related art, since a slicer performs binarization in the case of a reproduced signal exhibiting a large degree of asymmetry, difficulty is encountered in decoding a 2T signal. Thus, an essential resolution has not yet been achieved.